DANVILLE, Ind. (AP) -- A landowner whose property includes wetlands used by the endangered Indiana bat is threatening to sue in a bid to halt the final leg of a $4.9 billion natural gas pipeline.

The eastern portion of the Rockies Express Pipeline, which runs from northwest Colorado to Ohio, will cross Dave McCarroll's 60 acres near Danville just west of Indianapolis that includes forested wetlands the bats use in the summer.

McCarroll has filed a letter of intent to sue, contending that the project would destroy habitat used by the bat, which is one of the nation's most endangered terrestrial mammals.

Populations of the bat, which is about the size of a thumb, have fallen nearly 60 percent since it was listed as endangered in 1967.

A portion of the 639-mile pipeline will cross nine Indiana counties -- each of which harbors Indiana bat populations.

Aside from a possible lawsuit, about two dozen bat habitat surveys must be completed as part of a federal environmental document needed before construction can start in early June.

"If it's a highway or a pipeline, it doesn't matter," said Rob Mies, director of the Bloomfield Hills, Mich.-based Organization for Bat Conservation. "When their habitat is taken away, your chances of those bats finding a new place are slim."

Addressing the project's effect on the bat is one of 149 environmental measures the Federal Energy Regulatory Commission, which oversees the project, has placed on the pipeline before final approval is granted.

Pipeline officials stand by their environmental record and say the project needs to go ahead to tap into gas-rich areas in Colorado and Wyoming.

"It's important that as we try to rely more on clean sources of energy, this natural gas pipeline will transport natural gas from where it's plentiful throughout the country, including Indiana," said Allen Fore, a spokesman for Rockies Express Pipeline.

Four Indiana gas companies will tap into the pipeline, which is being developed by Rockies Express Pipeline LLC and three other companies.

In November, federal regulators issued a draft environmental impact statement that determined the pipeline was an "environmentally acceptable action," although Rockies Express East "is likely to adversely affect" the Indiana bat population.

Ongoing talks between U.S. Fish and Wildlife Service officials and the federal energy commission could lead to agreements on how to protect the Indiana bat and alter the draft report's conclusions.

Robert R. Clark, the attorney representing McCarroll, said the draft statement ignores the Endangered Species Act and jeopardizes the Indiana bat population.

In a Feb. 8 letter, Clark notified the pipeline, also know as REX-East, and five federal agencies that McCarroll intends to seek an injunction requiring them to "follow federal law and preserve the endangered species and habitat" on his property.

Surveyors hired by McCarroll found 10 Indiana bats, including six lactating females, on his property -- evidence that led a scientist to determine the likely existence of a maternal colony.

Clark argues the pipeline's aggressive schedule denies the commission enough time to assess the project's environmental effect without additional surveys.

Last week, McCarroll agreed to allow surveyors on his property. He and his attorney have recommended an alternate path that McCarroll says would avoid the bat habitat and add less than a quarter of a mile to the pipeline's route.